A. Field of the Invention
The invention relates generally to coding, and, more particularly, to a coding scheme that utilizes a multi-level signaling technique, such as PAM-4, to send and receive data and control information over a serial transmission medium.
B. Description of the Related Art
In modern communication systems, unidirectional high-speed serial links are often employed for communication between chips on a board or between boards in a modular system. In this regard, previous and current generation high-speed serial links have almost unanimously employed 2-level NRZ-signaling (Non-Return-to-Zero signaling).
However, as the speed of serial communication links increase into the Gigabit range, designers are increasingly considering abandoning NRZ-encoding on serial links in favor of multi-level signaling, because it reduces the fundamental frequency that is needed to traverse the transmission medium. One such common multi-level signaling method is 4-level signaling, also known as 4-PAM or PAM-4.
U.S. Patent Publication No. 2003/0108134, to Stonick et al., describes a method and apparatus for encoding and decoding digital communication data using PAM-n techniques. In the Stonick et al. reference, transitions between the two most extreme signal levels are eliminated in the encoded sequences.
U.S. Patent Publication 2004/0109509, to Stonecypher et al., describes a technique for improving the quality of digital signals in a multi-level signaling system, whereby sets of N bits are encoded to provide corresponding sets of P symbols, and whereby each of the sets of P symbols are selected to eliminate full-swing transitions between successive digital signal transmissions.
U.S. Pat. No. 4,486,739, to Franaszek et al., describes a byte-oriented DC balancing technique that uses an 8b/10b coder that is partitioned into a 5b/6b plus a 3b/4b coder. The 8b/10b coding scheme is a commonly employed scheme in present day communication systems.
Before being transmitted on a high-speed serial link, data needs to be encoded to obtain certain properties required for reliable communication:                1. A certain bit transition density is required to ensure the receiving clock-data-recovery circuitry is capable of extracting a suitable receive clock from the bit stream.        2. Since data is presented to the receiver as a continuous stream of bits, the receiver needs some information to be able to infer the boundaries of the data words in the bit stream.        3. As most high-speed serial links operate AC-coupled, i.e., they employ a DC-blocking cap in the path between a transmitter and a receiver, the transmitted bit stream needs to be DC-balanced.        4. On a high-speed serial link employing multi-level signaling, it may be advantageous to eliminate any direct transitions between the outer signaling levels, as this will reduce the maximum voltage swing and the maximum slope of the transmitted signal, leading to reduced frequency components and less reflections and crosstalk.        
For high-speed serial links using NRZ signaling, the 8b/10b coding scheme, such as the one described in Franaszek et al., has provided a coding scheme that provides the properties #1–#3 (as property #4 is not applicable for such links).
However, the 8b/10b coding scheme is not suitable for use on high-speed serial links using 4-PAM signaling, since on such a link the 8b/10b coding scheme does not guarantee a high transition density or DC-balancing. The 4S/5S coding scheme described in Stonecypher et al. also does not provide for DC-balancing. The problems of clock data recovery for a PAM-4 system are described in “Equalization and Clock Recovery for a 2.5–10 Gb/s 2-PAM/4-PAM Backplane Transceiver Cell,” which reference describes an equalization architecture to handle intersymbol interference.
Hence, a new coding scheme is needed that suits the special requirements of a 4-PAM, 8-PAM, or other type of multi-level encoded link with regards to the above requirements.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.